Circulating Bmp10 acts through endothelial Alk1 to mediate flow-dependent arterial quiescence

Abstract

SUMMARYBlood flow plays crucial roles in vascular development, remodeling and homeostasis, but the molecular pathways required for transducing flow signals are not well understood. In zebrafish embryos, arterial expression of activin receptor-like kinase 1 (alk1), which encodes a TGFβ family type I receptor, is dependent on blood flow, and loss of alk1 mimics lack of blood flow in terms of dysregulation of a subset of flow-responsive arterial genes and increased arterial endothelial cell number. These data suggest that blood flow activates Alk1 signaling to promote a flow-responsive gene expression program that limits nascent arterial caliber. Here, we demonstrate that restoration of endothelial alk1 expression to flow-deprived arteries fails to rescue Alk1 activity or normalize arterial endothelial cell gene expression or number, implying that blood flow may play an additional role in Alk1 signaling independent of alk1 induction. To this end, we define cardiac-derived Bmp10 as the crucial ligand for endothelial Alk1 in embryonic vascular development, and provide evidence that circulating Bmp10 acts through endothelial Alk1 to limit endothelial cell number in and thereby stabilize the caliber of nascent arteries. Thus, blood flow promotes Alk1 activity by concomitantly inducing alk1 expression and distributing Bmp10, thereby reinforcing this signaling pathway, which functions to limit arterial caliber at the onset of flow. Because mutations in ALK1 cause arteriovenous malformations (AVMs), our findings suggest that an impaired flow response initiates AVM development.

Alk1 controls arterial endothelial cell migration in lumenized vessels

“…Because our previous data demonstrated that lack of blood flow mimics lack of alk1 with respect to changes in arterial endothelial cell number and gene expression (Corti et al, 2011;Laux et al, 2013), we also performed photoconversion experiments in tnnt2a morphants, which lack heartbeat and blood flow (Sehnert et al, 2002). In the absence of blood flow, we documented decreased proximal displacement and increased distal displacement of alk1-positive endothelial cells between 24 and 48 hpf, compared with controls and alk1 morphants (Fig.…”

“…Arterial endothelial cells that contribute to the contiguous AA1, ICA and CaDI derive from both of these clusters and become alk1 positive only after the onset of blood flow (Corti et al, 2011). We previously reported an increase in caliber of and endothelial cell number in the CaDI in 36 hpf alk1 morphant embryos compared with control siblings (Laux et al, 2013). To define the time course of these changes and to determine whether other alk1-positive arteries behave in a similar manner, we assessed morphology of and counted endothelial cells in proximal (AA1 and ICA; Fig.…”

“…1). We previously reported that blood flow is required for alk1 expression, and that Alk1 transmits a blood flow-dependent signal that limits cell number in and caliber of the CaDI (Corti et al, 2011;Laux et al, 2013). In alk1 mutants, AVMs develop downstream of the enlarged CaDI, connecting the basal communicating artery (BCA) to the primordial midbrain channel (PMBC) or the basilar artery (BA) to the primordial hindbrain channel (PHBC) (Fig.…”

Myocardium and BMP signaling are required for endocardial differentiation

“…Multiple zebrafish BMP homologs, including BMP2b, BMP3, BMP4 and BMP10l, are known to have expression in the myocardium (Chin et al, 1997;Wang et al, 2007;Schoenebeck et al, 2012;Laux et al, 2013). It is likely that two or more homologs may function redundantly during endocardial differentiation.…”

Alk1 controls arterial endothelial cell migration in lumenized vessels

“…Zebrafish alk1 mutants develop AVMs at a predictable time (approximately 40 h post-fertilization, hpf ) in a predictable location (beneath the midbrain or hindbrain) and therefore serve as a relevant, accessible model for exploring the cellular basis of HHT-associated AVM development (Corti et al, 2011;Laux et al, 2013;Roman et al, 2002).…”